The invention relates to a method and a system for testing an electrical component.
Electrical components, such as networks of printed circuit boards or integrated circuits, have to be tested with respect to their electrical properties. These measurements comprise, for instance, the capacitance, RC-constants, opens and/or shorts of the component.
A conventional method of capacitance measurement is the application of an electrical C-measurement bridge, in which the contact to the component to be measured is performed by mechanical probes.
It is also known to use a particle beam for contactless measurement of shorts and/or opens. In this technique a network is charged up and the potential distribution along the network and on adjacent networks is measured by contactless potential measurement techniques, e.g. by the energy analysis of released secondary particles. This technique gives detailed local information about shorts and opens, but a capacitance- or RC-measurement cannot be performed.
U.S. Pat. No. 5,602,489 describes a method for testing the interconnect networks of a multichip module for opens and shorts. It comprises the steps of:
maintaining an extract grid at a first grid potential;
defining a potential on a pad such that the pad and the extract grid reach a first state;
switching the extract grid to a second grid potential;
maintaining the extract grid and the second grid potential until the pad and the extract grid reach a second state; and
measuring a test time, in order to determine the capacitance of the interconnect network being tested.
Irregularities in the network (shorts to other networks or opens within one network) will change the capacitance and therefore the test time for reaching the second state. The observation of this charging and recharging process is performed by a detector, which detects the secondary particles passing the extract grid.
Although the method known from the U.S. Pat. No. 5,602,489 is much faster, because only one location of the component has to be measured, it also has some severe drawbacks which limit measurement accuracy and accordingly its applicability. Since the measurement merely relies on a time difference measurement, any variation in the beam current and secondary particle yield will also affect the time difference measurement from the first state to the second state. Beam current variations can be controlled by beam current measurement. Secondary yield changes, however, are difficult to detect. They can be caused by surface contamination, surface roughness, angle of irradiation and so on. Therefore, the measurement contains not necessarily information about the capacitance alone, but includes the effect of many other parameters. Consequently, the method is not reliable.
It is therefore an object of the invention, to provide a method and a system for testing an electrical component which is fast and provides reliable results.
The method for testing an electrical component according to the invention comprises the steps of:
positioning a primary particle beam onto the component,
supplying an AC-signal to an electrode being positioned in front of the component,
varying the frequency of the AC-signal,
detecting secondary particles released at the component and penetrating the electrode to form a secondary particle signal, and
evaluating the corresponding secondary particle signal.
It is preferred to evaluate the corresponding secondary particle signal with respect to an AC-amplitude variation and/or phase shift relatively to the AC-signal of the electrode.